“Air data products,” as they are called, are determined in an aircraft using an in-flight air data system. An air data system incorporates instrumentation to collect air data products, and supplies this data directly to an aircraft's flight computer for flight control purposes. Common air data products include, but are not limited to, Mach number, true airspeed, calibrated airspeed, vertical speed, static density, static air temperature, sideslip, angle of attack, pressure altitude, and dynamic pressure.
Perhaps the oldest type of such instrumentation is the Pitot static tube. The Pitot tube (named after Henri Pitot in 1732) measures a fluid velocity by converting the kinetic energy of the flow into potential energy. The conversion takes place at the stagnation point, located at the Pitot tube entrance. A pressure higher than the free-stream (i.e. dynamic) pressure results from the kinematic to potential conversion. This “static” pressure is measured by comparing it to the flow's dynamic pressure with a differential manometer.
Pitot static tubes have proven quite effective over the years; however, there are a number of characteristics that make them undesirable in some situations. For example, at high angles of attack the air data measurements provided by pitot static systems are significantly degraded. Pitot tubes also contribute significantly to an aircraft's radar cross section, since they protrude from the aircraft body. The installation and calibration of pitot static tubes must be tailored to each airframe, and airframe modifications may require recalibration of the air data system.
Optical air data system technologies are alternatives to the traditional pitot static system. In general, an optical air data system utilizes LIDAR (Light Detection and Ranging) to remotely analyze the atmosphere. LIDAR uses an active sensor that includes a laser light source, a detection system and an analysis routine to process the signal return.
There are two types of optical air data systems: coherent and direct detection (incoherent). In a coherent LIDAR, the laser light is emitted into the atmosphere, where it scatters off of the aerosols in the air, and can be analyzed to solely determine the air velocity. For these purposes, an aerosol is defined as any type of particle that is suspended in the air.
In a direct detection system, the laser energy scatters off of both aerosols in the air, as well as the air molecules themselves, and can be analyzed to determine the air velocity, density, and temperature.
A coherent LIDAR system utilizes relatively long wavelength laser energy and relies upon Mie scattering, which is the scattering of light off of the aerosols suspended in the air. More particularly, Mie scattering refers to the scattering of light off of particles greater than 1/10th the wavelength of light. However, since coherent detection LIDAR measures the properties of aerosols, it can only measure the wind velocity.
Because coherent LIDAR approaches rely solely on Mie scattering, they cannot make measurements in clean air where there are no aerosols present. In addition, coherent approaches typically utilize relatively long wavelength light, which is not absorbed by the atmosphere, presenting additional issues with long-range detection, and increased eye safety hazards.